Conventionally, in optical modulators that modulate light generated by a light source, a Mach-Zehnder interferometer may be provided. In those optical modulators, a signal electrode and a ground electrode are provided along parallel optical waveguides. In recent years, because optical modulation methods are diversified, each optical modulator is often provided with two or more Mach-Zehnder interferometers. In those situations, by integrating the two or more Mach-Zehnder interferometers on one chip, it is possible to keep the size of the optical modulator small.
An optical modulator provided with two or more Mach-Zehnder interferometers is able to generate multi-level modulation signals by having a plurality of mutually-different electrical signals input thereto. In other words, by having the mutually-different electrical signals input from an external source to signal electrodes corresponding to the different Mach-Zehnder interferometers, the optical modulator is able to perform an optical modulation process that uses a multi-level modulation method such as a Differential Quadrature Phase Shift Keying (DQPSK) method.
In an electrical signal input section of an optical modulator, connectors corresponding to different electrodes may be provided individually; however, when one connector is provided for each of a plurality of electrical signals, the size of the optical modulator becomes large, and the mounting area increases. To cope with this situation, examples of methods for keeping the apparatus compact includes configuring the electrical signal input section by using a Flexible Printed Circuits (FPC) unit that has flexibility.
More specifically, the FPC unit has a plurality of wiring patterns corresponding to the plurality of signal electrodes of the optical modulator printed thereon, so that the electrical signals output from the driver are input to the optical modulator via the wiring patterns printed on the FPC unit. As for positional arrangements of the wiring patterns printed on the FPC unit, a coplanar path pattern is known, for example. When a coplanar path pattern is used, as illustrated in FIG. 12, for example, a signal-purpose wiring pattern 20 is printed on a surface of a substrate 10, and ground-purpose wiring patterns 30 are printed on either side of the signal-purpose wiring pattern 20. In other words, in correspondence with one signal electrode, the one signal-purpose wiring pattern 20 and the two ground-purpose wiring patterns 30 are printed on the FPC unit. Further, when the entirety of the FPC unit having the wiring patterns printed thereon is inserted into a connector provided for an optical modulator or a driver for example, the wiring patterns provided on the FPC unit are electrically connected to electrodes of the optical modulator or the driver.
Patent Document 1: Japanese Laid-open Patent Publication No. 2013-29791
Patent Document 2: Japanese Laid-open Patent Publication No. 2010-28800
However, connecting an FPC unit and a connector to each other presents a problem where it is difficult to achieve impedance matching by arranging the characteristic impedance of each of the wiring patterns formed on the FPC unit to be 50Ω. In other words, when the wiring patterns are printed by using a coplanar path pattern as described above, in order to arrange the characteristic impedance at the connection between the FPC unit and the connector to be 50Ω, the gap between the signal-purpose wiring pattern and the ground-purpose wiring pattern becomes as small as approximately tens of micrometers. For this reason, even by a very small variation during the manufacture of an FPC unit, for example, a short circuit may be caused among the wiring patterns when the FPC unit is inserted into a connector, or an impedance mismatch may be caused due to the characteristic impedance being other than 50Ω.
To cope with this situation, one possible idea that can be used for enlarging the gap between the signal-purpose wiring pattern and the ground-purpose wiring pattern is to use a grounded-coplanar-type path pattern. When the grounded-coplanar-type path pattern is used, a ground electrode is provided on the entirety of the rear surface on the back of a substrate on which the signal-purpose wiring pattern and the ground-purpose wiring patterns are formed. By using this configuration, it is possible to reduce the impact caused by the level of precision of the gap formed between the signal-purpose wiring pattern and the ground-purpose wiring pattern.
However, in order to arrange the characteristic impedance to be 50Ω while using the grounded-coplanar-type path pattern, the width of the signal-purpose wiring pattern becomes as small as approximately tens of micrometers. Consequently, a problem arises where, when the FPC unit is inserted into a connector, a contact failure may occur between the wiring patterns provided on the FPC unit and terminals provided inside the connector, or the signal-purpose wiring pattern may be detached from the substrate. Consequently, it is not realistic to use the grounded-coplanar-type path pattern at the part where the FPC unit is inserted into the connector.
As explained above, when the optical modulator and the driver are connected by using the FPC unit, it is difficult to achieve the impedance matching by arranging the characteristic impedance of each of the wiring patterns formed on the FPC unit to be 50Ω.